Accurately and precisely locating a communications terminal (e.g. a vehicle) can be necessary for a variety of reasons. As is well known, the task of doing this can be difficult. It becomes even more difficult when the communications terminal is located underwater (e.g. in an undersea environment). Typically, present day systems for precisely locating objects rely primarily on radio waves such as are employed by Global Positioning Systems (GPS). Radio waves, however, are essentially impenetrable into an undersea environment.
Unlike radio waves, under certain circumstances and conditions, light waves can penetrate and propagate through water; at least to some extent. Of particular importance is the demonstrated ability of light to penetrate into seawater when the light has a wavelength in the so-called blue-green seawater window (e.g. λ≈450-500 nm). Importantly, there are indications that light with a wavelength λ≈455 nm can penetrate seawater to a depth d in excess of 40 m. With this in mind, the possibility of replacing radio waves with light waves for the purpose of passively locating an underwater terminal deserves consideration.
An important consideration is that the daylight operation of an optical Global Positioning System (oGPS) must necessarily account for the solar background, which can be substantial and significantly reduce the Signal-to-Noise Ratio (SNR). For very high noise environments such as solar background, it is therefore necessary to eliminate as much noise as possible. To do this, several types of optical filters exist which only transmit light over a very narrow wavelength bandwidth, such as Lyot filters and atomic line filters. For example, U.S. Pat. No. 5,731,585, which issued to Menders et al. on Mar. 24, 1998, for an invention entitled “Voigt Filter,” discloses a kind of atomic line filter which can operate at the 455 nm cesium resonance wavelength.
In light of the above, it is an object of the present invention to provide a system and method wherein light pulses are radiated from a transmitter in near space (e.g. outside the atmosphere) to a terminal (receiver) within a liquid medium (e.g. undersea), for use at a terminal to accurately and precisely determine the terminal's position in the liquid medium. Another object of the present invention is to provide a system and method for passively locating an underwater terminal wherein light pulses are simultaneously transmitted from different geosynchronous satellites onto a same cell area of a terrestrial water surface, for a use of light pulse arrival times at the underwater terminal to establish the underwater terminal's terrestrial location. Still another object of the present invention is to provide a system and method for passively locating an underwater terminal using light pulses which is simple to use, relatively easy to implement, and comparatively cost effective.